The present invention relates mainly to a master cylinder for an electro-hydraulic braking system comprising improved means of simulating the pedal feel and to an electro-hydraulic braking system comprising such a master cylinder.
Electro-hydraulic braking systems comprise a master cylinder which, in normal operation, simulates the mechanical reaction of a conventional braking circuit felt at the brake pedal by a driver, and means for detecting the action of the driver on the brake pedal sending the information to a computer which generates the command to a hydraulic pump to send pressurized brake fluid to the brakes. In degraded operation, for example when the pump does not respond, the master cylinder supplies pressurized brake fluid to the brakes like in a conventional braking circuit.
Master cylinders for electro-hydraulic braking systems of known type comprise a body of substantially cylindrical shape in which there is made a bore divided into at least one feed chamber and one working chamber by a piston mounted to slide in a sealed manner in the bore and actuated by an actuating rod connected to a brake pedal. The piston at rest allows communication between the two chambers and sealedly separates the two chambers during a braking action. The supply chamber is connected in a sealed manner to a brake fluid reservoir and the working chamber in normal operation is connected to a cartridge simulating the pedal feel or pedal feel cartridge and in degraded operation is connected to at least one brake arranged at a wheel.
The pedal feel cartridge comprises an envelope delimiting a substantially cylindrical chamber in which there slides a piston subjected, in normal operation, during a braking phase, via a first face, to the pressurized brake fluid supplied by the working chamber and via a second face to a first end of an elastic means, the second end of the elastic means bearing against the closed end of the chamber opposite the piston. The elastic means makes it possible to simulate the mechanical reaction of a conventional braking circuit, which corresponds to a relationship connecting the force at the pedal as a function of the pedal travel. The characteristic curve of this relationship has at least one first part corresponding to the absorption of the braking circuit at the beginning of the braking phase, then a second part corresponding to a reaction which increases in magnitude as the level of braking increases.
The elastic means of known type used in pedal feel cartridges are therefore very complicated and expensive; for example, they contain several helical springs, with different spring rates, with constant pitch, variable pitch, elastomer elements to simulate the absorption of the circuit. In addition, assembly is lengthy and therefore increases the cost price because of the high number of components needed to correctly simulate a conventional pedal reaction.
Electro-hydraulic braking systems provide excellent control over the braking and the vehicle, although the means simulating the reaction of the braking circuit are relatively complicated and require precise adjustment, making it inconceivable to apply them to a wide range of vehicles.
In addition, motor vehicle manufacturers are wanting to standardize, as far as possible, the components that make up the braking systems. However, each type of motor vehicle has a characteristic pedal feel, which currently entails making significant modifications to the pedal feel simulation means to suit each type of motor vehicle.
It will be particularly advantageous for the driver of the vehicle to be able to modify the pedal feel according to the desired type of driving, something which is currently impossible with the existing means. In particular, when the motor vehicle is driven by several people who all have different braking preferences, it would be possible for the vehicle computer to store the settings of the pedal feel device relating to each person likely to use the motor vehicle, and to carry out automatic setting-up once the driver has been identified.
It is conceivable to reduce the number of parts of which the simulation cartridge is formed, for example using just one spring.
The use of springs of the helical type with uniform pitch is unsuitable because these springs have a constant spring rate k, and the deformation force Fd is connected as a linear relationship to the axial deformation x by the relationship Fd=k*x, this relationship being valid as long as the spring deformation is elastic.
There are also variable-pitch springs the deformation force of which is a non-linear function of the axial deformation of the spring and the characteristic of which is more similar to that of a conventional braking circuit, although these springs do not allow adjustment of the simulated pedal feel to suit the type of vehicle and/or the driver.
Thus, one object of the present invention is to offer a master cylinder for an electro-hydraulic braking system that is of simple design and allows a simulated pedal feel very similar to the reaction of a conventional braking circuit.
Another object of the present invention is to offer a master cylinder for an electro-hydraulic braking system, that can easily be applied to various models of motor vehicle.
Another object of the present invention is to offer a master cylinder for an electro-hydraulic braking system, the simulated pedal feel of which can be adjusted in a simple way.
Another object of the present invention is to offer a master cylinder for an alectro-hydraulic braking system, that can be used for several models of motor vehicle.
These objects are achieved by a master cylinder comprising a cartridge simulating the pedal feel comprising an elastic means reproducing the reaction of a conventional braking circuit, the elastic means comprising a helical spring and a spacer means arranged between at least two consecutive turns of the helical spring bearing two axially opposed bearing surfaces for the two turns and altering the stiffness of the elastic means when the turns come to rest against the spacer means.
In other words, the elastic means comprises a spring with several turns, coupled to a spacer piece inserted between several turns of the spring, thus forming pairs of turns of the spring which are separated by a spacing turn; as the load on the spring increases, the turns of the spring come gradually and continuously into contact with the spacing parts arranged between each pair of turns, causing a gradual and continuous increase in the stiffness of the elastic means and therefore a reaction at the brake pedal similar to the reaction of a conventional braking circuit.
How this works is that the stiffness of the elastic means determines the reaction of the cartridge simulating pedal feel, the stiffness of the elastic means being inversely proportional to the number of active turns in the helical spring, the turns moving axially in relation to the other turns when load is applied to the spring are termed the active turns.
In consequence, when the elastic means according to the present invention is placed under load, regions of turns of the spring come gradually into contact with the spacer means until the said regions of turns become fixed relative to the turns region following or preceding it in the direction in which the load is applied, the following or preceding turns region being a turns region that is deactivated and acts as a support for deactivating the active turns region.
Advantageously, the spacer piece has a variable cross section along the axis of the spring, and this therefore makes it possible to have a variation in the change in stiffness of the elastic means, offering a reaction very similar to a reaction of a conventional braking circuit.
In addition, the reaction felt at the brake pedal can be altered, by altering the relative position of the spacer means relative to the spring, it is therefore possible to provide a device making it possible, for example, to alter the variation in stiffness of the elastic means, advantageously controlled by a computer of the braking system.
The main subject of the present invention is a master cylinder for electro-hydraulic braking systems comprising a body of longitudinal axis pierced with a bore, a piston mounted to slide in sealed fashion in the bore and axially dividing the bore into a supply chamber sealedly connected to a brake fluid reservoir and a working chamber, the piston being operated by an actuating rod connected to a brake pedal, and a cartridge simulating the reaction of a braking circuit at the pedal, the working chamber in normal operation being in communication with the inside of the cartridge simulating the pedal feel and in degraded operation being in communication with at least one brake arranged at a wheel, the cartridge comprising a piston able to be subjected, via a first face to the pressurized brake fluid supplied by the working chamber and subjected via a second face to a variable-stiffness elastic means, characterized in that the elastic means comprises at least one helical spring and a spacer means arranged between at least two turns of the spring and allowing the said turns to press against the spacer means when the pressure of the brake fluid in the working chamber exceeds a predetermined value.
Another subject of the present invention is a master cylinder, characterized in that the spacer means is formed of a helix forming turns.
Another subject of the present invention is a master cylinder, characterized in that the helix of the cartridge has a cross section of which the dimension along the longitudinal axis continuously increases or decreases by helix portions.
Another subject of the present invention is a master cylinder, characterized in that the spacer means is made of plastic.
Another subject of the present invention is a master cylinder, characterized in that the helical spring has a uniform pitch.
Another subject of the present invention is a master cylinder, characterized in that the spacer means comprises a second longitudinal end mounted fixedly with respect to the piston of the cartridge.
Another subject of the present invention is a master cylinder, characterized in that the cartridge comprises means of adjusting the simulated pedal feel by altering the axial position of the spacer means relative to the spring.
Another subject of the present invention is a master cylinder, characterized in that the adjusting means comprise a stepping electric motor allowing the spacer means to be moved longitudinally and rotationally.
Another subject of the present invention is an electro-hydraulic braking system comprising means of detecting the braking action of the driver, a computer receiving the information from the detection means and generating commands to actuate at least one brake arranged at a wheel, a pressure generator receiving the command from the computer to send pressurized fluid to the brakes, a master cylinder allowing the pedal feel to be simulated under normal operation and serving as a source of pressurized brake fluid in degraded operation, and electrically operated valves to interrupt the communication between the said master cylinder and the brakes in normal operation, characterized in that the said master cylinder is a master cylinder according to the invention.
Another subject of the present invention is a braking system, characterized in that the pressure generator is an electric pump.
The present invention will be better understood with the aid of the description which follows and of the appended drawings for which the front, the rear, the upper part and the lower part correspond respectively to the left, the right, the top and the bottom of the drawings.